1. Field of the Invention
The present invention relates to a solder resist ink composition used in a printed circuit and, more particularly, to a solder resist ink composition containing a photo-curable resin having a polymer skeleton of a novolak-type epoxy resin.
2. Description of the Prior Art
In order to protect a printed circuit and to prevent the unnecessary attachment of solder to portions of a printed circuit board other than those into which circuit components are soldered, a protective film pattern composed of a solder resist ink is formed on a surface of the circuit board.
According to a typical conventional method, a solder resist ink is screen-printed to form a protective film pattern on a circuit board. However, with recent increases in the packing density of printed circuits, use of a photoetching method has been investigated, for forming a protective film micropattern. As a result, a photo-curable solder resist ink is being developed for the purpose of photoetching. When forming a protective film pattern by means of photoetching, a photo-curable solder resist ink composition is applied to the surface of a printed circuit. Only a predetermined portion of the resist film is exposed light, by means of which it is cured. The nonexposed portion of the solder resist ink composition is dissolved and removed by a developing solution, whereby a desired protective film pattern is obtained.
Examples of the conventional photo-curable solder resist are compositions which contain photo-curable resins as main components and which are prepared by causing a compound containing a photo-sensitive group to react with an epoxy resin, as is described in Japanese Patent Disclosures Nos. 58-62636, 60-208377, and 61-272.
These photo-curable solder resist compositions, however, are required to use, as a developing agent, a flammable organic solvent prepared by mixing an alcohol- or ether-based solvent in cyclohexanol or 1,1,1-trichloroethane, or a chlorine-containing organic solvent. Use of these solvents pollutes the atmosphere and is detrimental to workers involved in the fabrication process. For this reason, special equipment must be provided to keep the atmosphere clean, which results in increased costs.
A conventional solder resist ink composition containing a cresolnovolak-type epoxy resin or a phenolnovolak-type epoxy resin as a main component has been proposed as a composition which instead uses an aqueous alkaline solution as the developing solution (Japanese Patent Disclosure No. 61-243869).
The solder resist ink composition using the cresolnovolak-type epoxy resin has good thermal resistance, a high degree of hardness, and good electrical characteristics and is free from stickiness during preliminary drying. However, there are also disadvantages in that the time required for photo-curing reaction is long and its development performance is low.
The solder resist ink composition using the phenolnovolak-type epoxy resin, on the other hand, has good photo-curing reaction and developing properties, but has poor heat resistance and electrical characteristics, low hardness, and has a sticky texture during preliminary drying.